As for the headlights mounted on a vehicle, to improve safety during night traveling as well as providing high designability and luxuriousness, instead of conventional halogen lamps, bright discharge lamps or LEDs (light emitting diodes) capable of illuminating an area in any desired direction have been widely used as light sources.
In order to mount the above bright light sources on a vehicle, when the rear of the vehicle is lowered, for example, because a passenger takes a rear seat or baggage is brought into the trunk, that is, when the illumination directions of the headlights tilt upward because the front of the vehicle is elevated, it is necessary to lower the illumination directions of the headlights, that is, necessary to lower the optical axes of the headlights so as to maintain the optical axes with respect to a road surface to prevent a driver of an oncoming vehicle from being dazzled and to prevent pedestrians facing the headlights from being made uncomfortable. In short, it is essential for the vehicle using the above bright light sources to have the headlight optical-axis control device for lowering the illumination directions of the headlights to thereby return it to an illumination direction that was used before its change, when the illumination directions of the headlights tilt upward owing to the tilt of the vehicle caused because a passenger gets on the vehicle or baggage is taken in the trunk.
In this regard, the passenger gets on the vehicle or baggage is taken in the trunk when the vehicle is stopped. The optical-axis control when the vehicle is stopped is the main control of the headlight optical-axis control device.
Since the optical-axis control of the headlights moves the optical axis up or down to compensate for the change of the tilt angle of the vehicle with respect to the road surface to thereby return the illumination direction of the headlights to the original direction where the vehicle tilts in the longitudinal direction as described above, it is first necessary to measure the tilt angle of the vehicle with respect to the road surface.
In a conventional system, using stroke sensors mounted on suspensions (suspension system) at both the front and rear of the vehicle, the shrinkages of the suspensions at the front and rear, that is, the lowering amounts of axles in the front and rear are measured, and, then, the tilt angle of the vehicle with respect to the road surface is calculated on the basis of the length of its wheelbase and the difference between the lowering amounts.
Recently, besides the configuration using the stroke sensors mounted on the above suspensions, a configuration using an acceleration sensor has been researched as disclosed in patent literature 1. In the configuration using the acceleration sensor, it is easy to detect a change in the tilt angle of the vehicle when the vehicle is stopped, and it is easy to obtain the tilt angle at the current moment by accumulating the changes relative to an initial tilt angle, caused when a passenger gets on and off the vehicle or in similar situations. On the other hand, the output of the acceleration sensor contains an offset and changes in the offset over time, and the tilt angle obtained by the accumulation includes a latent cumulative error. Thus, there is the problem that the accuracy of the tilt angle obtained by accumulating detection values and the changes is low. For this reason, to stably maintain the optical axes of the headlights at a proper angle over a long time, it is necessary to maintain the accuracy by applying some correction to accelerations detected by the acceleration sensor.
The above-described optical-axis control device of patent literature 1 improves the accuracy of the tilt angle of a vehicle with respect to a road surface by using a biaxial acceleration sensor for the longitudinal and perpendicular directions of the vehicle, and does not merely carry out an optical-axis control of the headlights when the vehicle is stopped, but also detects accelerations to carry out the optical-axis control of the headlights when the vehicle is traveling, to thereby carry out suitable optical-axis control of the headlights. The above-described optical-axis control device of patent literature 1 calculates the tilt angle of the vehicle with respect to the road surface by obtaining the changed direction of an acceleration for each time using the acceleration that is detected when the vehicle is traveling, or by obtaining the changed direction of the acceleration from two accelerations at different detection timings. The optical-axis control device controls the optical axis on the basis of the changes of the calculated tilt angle with respect to the road surface.